In material testing, tests with dynamic mechanical load changes on test bodies are widely known. For this, widely varying types of test machine are used with different working methods, wherein as well as mechanical systems, in particular three drive types are distinguished: namely hydraulic systems, ultrasound systems and electromagnetic systems.
Hydraulic systems achieve a working frequency, i.e. a load frequency to be transmitted to a specimen, of less than 1000 Hz and require a high energy input, since an oil pressure controlled with working frequency must move a working piston. Furthermore, hydraulic systems usually require multistage valve controls which often necessitate high maintenance costs.
Ultrasound systems, i.e. oscillatory systems with an ultrasound emitter, typically achieve 15,000 to 20,000 load changes per second. Thus systems excited with ultrasound indeed have a high working frequency, but these high frequencies lead to often unacceptable heating of the test specimen, whereby the material tests must usually be carried out intermittently, i.e. with interruptions for intermediate cooling for the test specimen. In such material testing, however, it is often unclear how the load changes should be evaluated at the start and end of an interval. Due to the cooling intervals, the duration of the material test—despite the high working frequency—is greatly increased. Furthermore, for physical reasons, not all specimen forms can be tested with this method. Also the material stress in the test body is measured indirectly.
Electromagnetic systems are generally operated in resonance and usually excited with one or more electromagnets with working frequencies (base frequency of natural vibration) of typically 30 to 250 Hz, i.e. load changes per second. Such systems work as mechanical resonators with electromagnetic excitation.
A mechanical resonance testing machine with electromagnetic excitation is described in DE A1 31 02 778. This resonance testing machine has a seismic mass, a frame established thereon with a horizontal cross member, a vibration body held sprung by means of a pretension spring from the cross member, and a vibration exciter held between the vibration body and the test body. The test body is arranged between the vibration body and the seismic mass. Furthermore, the resonance test machine has a central threaded spindle between the cross member and the pre-tension spring, in order to overlay a static stress over the vibration stress exerted on the test body during the vibration strength test. The vibration exciter is fitted with two parts which are moveable relative to each other for vibration excitation, and of which one is connected to the vibration body and the other to a holding element for the test body. The two vibration exciter parts are connected together by rod-like elastic elements, the length of which is dimensioned such that an air gap is present between the two vibration exciter parts under all conceivable operating conditions, allowing vibration excitation with good efficiency.
The resonance test machine according to DE A1 31 02 778 is also described as a three-mass resonance test machine containing the vibration body, the mass of the vibration exciter part connected to the holding element, and the seismic mass. The vibration body, the vibration exciter with the two vibration exciter parts and the elastic elements, the test body and the seismic mass, all lie in one axis, namely in a normal to the base surface. In this arrangement, the spring constant of the system determining the resonant frequency arises from the spring constants of the elastic elements of the vibration exciter and that of the test body. The spring constant f of a spring bar is generally proportional to the modulus of elasticity E of the spring bar, i.e. f=E·A/lo, wherein A is the cross sectional area of the spring bar and lo the mechanically unloaded length of the spring bar.
For the fatigue test of materials in industry and material research, today load change figures of the order of a few dozen (low cycle fatigue or LCF) up to multiples of 106 (high cycle fatigue or HCF) are required. Sometimes even load change figures up to 109 (very high cycle fatigue or VHCF) are required. In order to carry out material tests within an economically or technically acceptable test duration therefore the working frequencies must be increased. Previously known systems cannot, however, easily be adapted to modern requirements with regard to the significantly higher working frequencies.